Booleans are all messed up
Apply scale and recalculate Normals

The selected object has a negative scale. Despite you see it light grey in the viewport, the faces are indeed flipped for the modifiers, as the object transformations are runned after them.



I would suggest to apply the scale in Object mode (Ctrl+A), than go to edit mode and recalculate normals (Ctrl+N).




Result:



Notice that the same result can be achieved by simply give to the X scale a positive value, but to avoid misbehavior in future operations is better to fix the scale once for all: